The invention relates generally to an electrophotographic printing machine and, more particularly, to the non-interactive development of electrostatic images.
Generally, an electrophotographic printing machine includes a photoconductive member which is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to an optical light pattern representing the document being produced. This records an electrostatic image on the photoconductive member corresponding to the informational areas contained within the document. After the electrostatic image is formed on the photoconductive member, the image is developed by bringing a developer material into effective contact therewith. Typically, the developer material comprises toner particles bearing electrostatic charges chosen to cause them to move toward and adhere to the desired portions of the electrostatic image. The resulting physical image is subsequently transferred to a copy sheet. Finally, the copy sheet is heated or otherwise processed to permanently affix the powder image thereto in the desired image-wise configuration.
Development may be interactive or non-interactive depending on whether toner already on the image may or may not be disturbed or removed by subsequent development procedures. Sometimes the terms scavenging and non-scavenging are used interchangeably with the terms interactive and non-interactive. Non-interactive development is most useful in color systems when a given color toner must be deposited on an electrostatic image without disturbing previously applied toner deposits of a different color, or cross-contaminating the color toner supplies. This invention relates to such image-on-image, non-interactive development.
Apparently useful non-interactive development methods known to the inventor work by generating a powder cloud in the gap between the photoreceptor and another member which serves as a development electrode. It is generally observed that this gap should be as small as possible, as small as 0.010 inches or smaller. Generally, the larger the gap, the larger become certain image defects in the development of fine lines and edges. The lines do not develop to the correct width, lines near solid areas are distorted, and the edges of solids are softened, especially at corners. It is believed that these defects are due to arches in the image electric fields over lines and at the edges of solid areas. In these arches electric field lines from image charges loop up and return to the photoreceptor ground plane instead of reaching across through the cloud to the development electrode. Defects result because toner in the cloud moves generally along field lines and cannot cross them into the arches, with the result that the deposited toner distribution does not correspond to image charge distribution. Defects due to field arches are less serious in interactive two component development because toner is carried into the arches by carrier particles. Nor are they very serious in interactive single component development exemplified by U.S. Pat. No. 4,292,387 to Kanbe et al. because a strong, cross-gap AC field is superposed which overcomes the aforementioned field arch patterns.
In non-scavenging systems of the kind disclosed in the patents cited below, cross gap AC fields are also applied. However, it is important to realize that if such fields are made too strong, the system will become interactive due to toner impact on already developed images. Thus a system may image well at strong fields and develop non interactively at weak fields, but not do both simultaneously. The development electrode and its role in determining electric field structure is described, for example by H. E. J. Neugebauer in Xerography and Related Processes, Dessauer and Clark, Focal Press 1965. Powder cloud development is described, for example, in the paper xe2x80x9cHigh Sensitivity Electrophotographic Developmentxe2x80x9d by R. B. Lewis and H. M. Stark in Current Problems in Electrophotography, Berg and Hauffe, Walter de Gruyter, Berlin 1972.
U.S. Pat. No. 4,868,600 to Hays et al discloses a non-interactive development system wherein toner is first developed from a two-component developer onto a metal-cored donor roll and thereafter disturbed into a powder cloud in the narrow gap between the donor roll and an electrostatic image. Development fields created between the donor roll core and the electrostatic image harvest some of the toner from the cloud onto the electrostatic image, thus developing it without physically disturbing it. In this method the powder cloud generation is accomplished by thin, AC biased wires strung across the process direction and within the development gap. The wires ride on the toner layer and are biased relative to the donor roll core. The method is subject to wire breakage and to the creation of image defects due to wire motion, and these problems increase as the process width is increased. In this system it has been found important for image defect reduction to minimize the gap between the donor and the surface of the electrostatic image in order to create a close development electrode. Gap spacings of about 0.010 inches are characteristic. They would be smaller were it practical to maintain the necessary tolerances.
U.S. Pat. No. 4,557,992 to Haneda et al. describes a non-interactive magnetic brush development method wherein a two component employing magnetically soft carrier materials is carried into close proximity to an electrostatic image and caused to generate a powder cloud by the developer motion, sometimes aided by an AC voltage applied across the gap between the brush and the ground plane of the electrostatic image. Cloud generation directly from the surfaces of a two component developer avoids the problems created by wires. However, in practice such methods have been speed limited by their low toner cloud generation rate.
U.S. Pat. No. 5,409,791 to Kaukeinen et al. describes a non-interactive magnetic brush development method employing permanently magnetized carrier beads operating with a rotating multipole magnet within a conductive and nonmagnetic sleeve. Magnetic field lines form arches in the space above the sleeve surface and form chains of carrier beads. The developer chains are held in contact with the sleeve and out of direct contact with the photoreceptor by gradients provided by the multipole magnet. As the core rotates in one direction relative to the sleeve, the magnetic field lines beyond the sleeve surface rotate in the opposite sense, moving chains in a tumbling action which transports developer material along the sleeve surface. The strong mechanical agitation very effectively dislodges toner particles generating a rich powder cloud which can be developed to the adjacent photoreceptor surface under the influence of development fields between the sleeve and the electrostatic image. U.S. Pat. No. 5,409,791 assigned to Eastman Kodak Company is hereby incorporated by reference.
However, it has been observed that the use of bead chains according U.S. Pat. No. 5,409,791 requires that substantial clearance be provided in the development gap to avoid interactivity by direct physical contact between chains and photoreceptor. FIGS. 1 and 2, illustrates the rippled shape of the developer surface and the presence of bead chains. As a consequence of this clearance requirement the development electrode cannot be brought effectively close to the electrostatic image. With bead chains typical clearances are about 0.030 to 0.050 inches, whereas in a typical development system of the type described in U.S. Pat. No. 4,868,600 the gap between the donor and photoreceptor surface is brought down to about 0.010 inches. In devices according to U.S. Pat. No. 5,409,791 attempts to reduce the height of the developer mass by developer supply starvation have been found to result in a sparse brush structure of substantially the same height. Attempts to decrease the effective gap by increasing the electrical conductivity of the carrier have been partly successful. However, the open and stringy chain structure does not provide a very effective electrode material and problems remain, especially those related to image defects in lines and at edges.
U.S. Pat. No. 5,946,534, issued on Aug. 31, 1999 to applicant, the disclosure of which is incorporated herein by reference, discloses an apparatus and method for non-interactive, dry powder development of electrostatic images comprising: an image bearing member bearing an electrostatic image; two component developer comprising toner and permanently magnetized carrier beads, the carrier having average diameter (2a) and magnetization Mb a developer transporting member having a thickness t for transporting a developer layer of the two component developer, the layer spaced close to and out of contact with the image bearing member, and wherein the developer layer is substantially without chains of carrier beads, a multipole magnet member disposed in close proximity behind the transporting member and moving relative to it so as to sweep poles across its surface, the magnet member having a periodic magnetization of spatial frequency k and a peak magnetization M0 wherein Mb, t, k, and M0, are chosen such that Mb is sufficiently large to prevent the escape of developer, and a quantity C equal to:   2.2  ⁢      (                  M        0                    M        b              )    ⁢      ⅇ                  -        k            ⁢              xe2x80x83            ⁢      t        ⁢  k  ⁢      xe2x80x83    ⁢  a
is greater than about ⅓.
The present invention obviates the problems noted above by providing a non-interactive development system substantially without chains of carrier beads in the development zone, without fragile wires, and utilizing a cloud source of mechanically agitated, permanently magnetized carrier. Thus this invention is both robust and permits a spacing between a development electrode and the electrostatic image of about 0.010 inch, a spacing small enough to eliminate or significantly reduce image defects associated with fine lines and edges. This is accomplished by reducing bead-bead magnetic interaction relative to the interaction between individual beads and the field gradients applied by the multipole magnet.
There is provided apparatus for non-interactive, dry powder development of electrostatic images comprising: an image bearing member bearing an electrostatic image; two component developer comprising toner and permanently magnetized thickly coated carrier beads, said carrier beads having a radius r of the magnetized carrier bead core, a radius a of the coated carrier bead, and magnetization (Mb), wherein the ratio K of the radius r to the radius a is a quantity less than about xc2xe, a developer transporting member having a predefined thickness (t) for transporting a developer layer of said two component developer, said layer spaced close to and out of contact with said image bearing member, and wherein said developer layer is substantially without chains of carrier beads, a multipole magnet member disposed in close proximity behind said transporting member, and moving relative to it so as to sweep poles across its surface, said magnet member having a predefined periodic magnetization of spatial frequency (k) and a predefined peak magnetization (M0)
There is also provided a method for generating a substantially condensed developer layer on a developer roll, comprising the steps of assembling a developer magnetic assembly said magnetic assembly having a predefined periodic magnetization of spatial frequency (k) and a predefined peak magnetization (M0); enclosing the developer magnetic assembly with a sleeve of a predefined thickness (t) to form said developer roll; loading said developer roll with a single developer layer of two component developer comprising toner and permanently magnetized thickly coated carrier beads, said carrier beads having predefined average diameter (2a), a predefined magnetizable core diameter 2r, the ratio r/a being defined as K, and core magnetization (Mb); selecting said predefined thickness (t), said predefined periodic magnetization of spatial frequency (k), said predefined peak magnetization (M0), a predefined periodic magnetization of spatial frequency (k) and a predefined peak magnetization (M0), to satisfy the following relationship: wherein Mb, t, k, and M0, are chosen such that Mb is sufficiently large to prevent the escape of said developer, and that a quantity   C  =      2.2    ⁢          (                        M          0                          M          b                    )        ⁢          1              K        3              ⁢          ⅇ                        -          k                ⁢                  xe2x80x83                ⁢        t              ⁢    k    ⁢          xe2x80x83        ⁢    a  
is greater than about ⅓; and wherein the ratio K of the radius r of the carrier bead core to the radius a of the coated carrier bead is a quantity less than about xc2xe, and preferably less than about xc2xd, so as to weaken bead-bead interaction and thus enhance the desired provision of a compressed developer material layer.